It can be a mix, but the majority I've found is women. No worries! Have him go with you, then if he's uncomfortable (though I'm not sure why it would matter even if he's the only guy) then he can head to the beach and you stay in the class.
My husband goes to all the yoga and pilates classes he can...sometimes both the morning and afternoon class. So if you are going to be a CSA at the beginning of December, no worries, your bf won't be the only one. However, on the very few occasions I have gone with him, there have been no other guys.